1. Field of the Invention
The invention relates to a central transmission device of a four-wheel steering system, and more particularly to a mechanical type central transmission device for a four-wheel steering system.
2. Description of the Related Art
The concept of four-wheel steering (abbreviated as 4WS) has long been applied to the structures of the farming vehicles such as tractors, mowers, harvesters, etc., to facilitate movements in narrow, muddy and uneven farmland and country roads. The concept of four-wheel steering is also applied to passenger vehicles. When a 4WS vehicle is driven at a low speed, or steered to turn by a large angle, the front and rear wheels are turned in opposite directions to reduce the turning radius and to increase the vehicle's agility for roadside parking and turning. When the vehicle is driven at high speed or steered to turn by a small angle, the front and rear wheels are turned in the same direction to reduce lateral swinging of the vehicle body and to increase stability when changing driving lanes and driving at high speed.
Generally speaking, a four-wheel steering system primarily includes a front wheel turning mechanism, a rear wheel turning mechanism and a central transmission device connecting the front wheel turning mechanism and the rear wheel turning mechanism. The central transmission device includes a mechanical type and an electronic type. Since the mechanical-type central transmission device has better durability, better stability, and impact resistance, and is less susceptible to damage from ambient environmental conditions, the mechanical type central transmission device is relatively popular in research and development.
A mechanical type central transmission device can receive an angular displacement input from the front wheel turning mechanism and transmit a subsequent displacement output to the rear wheel turning mechanism. The structures and operating mechanisms of central transmission devices can be categorized into a swinging arm type (as disclosed in U.S. Pat. No. 4,572,316 and U.S. Pat. No. 5,341,294), a lever linkage type (as disclosed in U.S. Pat. No. 4,105,086), a cam types (as disclosed in U.S. Pat. No. 4,957,183), a gear type (as disclosed in U.S. Pat. No. 5,307,891), and other types (as disclosed in U.S. Pat. No. 5,667,032). However, all the foregoing types of central transmission devices suffer from the drawbacks of a complicated structure, a complex assembly, a large size, poor transmission reliability, difficulty in coupling with the present vehicle structures, etc. Accordingly, improvements may be made to the conventional four wheel steering system.